This invention relates to a radar phase measurement apparatus with a remote phase shifting element used for radar calibration.
Wide bandwidth coherent radars used in tracking systems, synthetic aperture measurement systems, and high range resolution systems measure relative phase differences at discrete frequencies between the transmitted and received signals. These differences are calibrated in terms of wavelength angle difference, which is referred to as phase angle. These phase measurements along with signal amplitude are transformed to the time domain via Fast Fourier Transforms (FFT), for range information. Range resolution R is inversely proportional to the number of discrete frequencies measured and the measurement bandwidth. High range resolution radars may require as much as 2,000 megahertz of bandwidth .
The phase delay through various components of the radar receiver varies as a function of the measurement frequency. The phase delay of the complete radar system must be calibrated at each measurement frequency across the receiver bandwidth.
Phase calibration is accomplished by measuring the radar phase angle output with a calibrated phase-shifted RF signal input.
In the prior art, phase shifting elements that are part of the radar system are used to vary the phase angle of a constant phase signal measured by or injected into the radar. This may be a waveguide phase shifter or a device at the radars intermediate frequency. Generally these devices shift the phase in increments of degrees, and the absolute accuracy is variable. These devices, especially the waveguide shifters, are inherently narrow band and the absolute accuracy degrades when they are used across a wide radio frequency (RF) bandwidth. The signal loss through component phase shifters may change as the phase angle is varied. This will introduce additional error in the overall absolute calibration of the radar system. Component phase shifters built into the radar system will introduce some additional losses that can degrade the overall sensitivity of the radar system. This will also introduce additional error in the overall absolute calibration of the radar system.